Optical isolators are one of the most ubiquitous of all passive optical components found in most optical communication systems. Optical isolators are generally used to allow signals to propagate in a forward direction but not in a backward direction. These isolators are often used prevent unwanted back reflections from being transmitted back to a signal's source. It is generally known that optical isolators are to some extent, wavelength dependent devices. They provide a greater amount of isolation for some wavelengths of light and less isolation for other input wavelengths of light.
One prior art polarization independent optical isolator is described in United States Pat. No. RE 35,575 issued Jul. 29, 1997 in the name of Pan and entitled Optical Isolator. Pan describes an isolator having an input fibre 17, an output fibre 18 wherein light is transmitted from the input to the output fibre is transmitted and wherein light propagating in a reverse direction from output to input is blocked. The optical isolator described has a glass ferrule 10 into which the input fibre 17 is inserted. The ferrule 10 helps align the fibre. Signals from the end of the input fibre are transmitted by a first GRIN lens 11 which collimates the light from the end of the fibre. The collimated light from the GRIN lens 11 is then passed through a polarizer 12 in the form of a birefringent crystal wedge. The polarizer separates the incident light from the GRIN lens into a ray polarized along the crystal's optical axis. The light from the polarizer is then rotated by a Faraday rotator 13 which rotates the polarized light by 45 degrees. The rotator is typically formed of garnet doped with impurities or, alternatively, YIG, placed in a permanent magnet.
A second polarizer 14 then recombines the rotated light. Like the first polarizer 12, the second polarizer 14 is formed by a birefringent crystal wedge. The optical axis of this birefringent crystal wedge. The optical axis of this birefringent crystal is oriented at 45 degrees with respect to the optical axis of the first polarizer. Thus the ordinary ray from the first polarizer is also the ordinary ray of the second polarizer and the extraordinary of the second polarizer. The net result is that after traveling from the first polarizer through the second polarizer, the two collimated rays are negligibly displaced from each other. The two rays are then combined and refocused by a second GRIN lens 15 to a point on the end of the output fibre. Again the end of the output fibre is aligned by a glass ferrule.
In the reverse direction, light from the output fibre 18 is separated by the polarizer 14 into two , an ordinary ray polarized along the axis of the polarizer 14, and an extraordinary ray polarized perpendicularly to the optical axis. When passing back through the Faraday rotator 13, the light in both rays is rotated 45 degrees. This rotation is non-reciprocal with the rotation of light in the forward direction, so that the ordinary ray from the second polarizer 14 is polarized perpendicularly with the optical axis of the first polarizer 12 and the extraordinary ray from the second polarizer 14 is polarized with the optical axis of the first polarizer 12. The ordinary and extraordinary rays from the second polarizer 14 have swapped places incident upon the first polarizer 12. Because of this exchange, the light having passed through the first polarizer 12, does not leave the polarizer 12 in parallel rays. The non-parallel light is focused by the GRIN lens 11 at points which are not located at the end of the input fibre 10. For a more detailed explanation of this type of optical isolator, see, for example, "Compact Optical Isolator for Fibers Using Birefringent Wedges," M. Shirasaki and K. Asomo, Applied Optics, Vol. 21, No. 23 December, 1982, pp. 4296-4299.
An isolated optical coupler is disclosed in U.S. Pat. No. 5,082,343 in the name of Coult et al. issued Jan. 21, 1992. The coupler described in the patent is comprised of a pair of lenses having a wavelength selective device and an isolator disposed therebetween.
Another optical isolator which attempts to improve upon Coult's design is described in U.S. Pat. No. 5,594,821 in the name of the applicant, Yihao Cheng, issued Jan. 14, 1997.
Yet another optical isolator is described in U.S. Pat. No. 5,267,078 in the name of Shiraishi et al.
It is well known that passing a signal through two isolators will provide additional isolation, or for that matter that a two stage isolator will provide more isolation than a same single stage isolator. And yet still further, a three stage isolator will provide more isolation for a wider band of wavelengths than a double stage isolator.
Notwithstanding, there are difficulties associated with making compact multistage isolators, for example having three stages. Simply duplicating the optical components used to fabricate a single stage isolator to make a double stage isolator is not economical and will not produce the most compact device. Hence, attempts have been made to lessen the number of components required to make a multi-stage isolator to fewer than two times the number of elements required to make a single stage isolator. For example, U.S. Pat. No. 5,237,445 in the name of Kuzuta discloses a three stage isolator which employs rutile (TiO.sub.2) as birefringent crystals and includes four rutile plates and three Faraday elements. One limiting aspect of Kuzuta's invention is that the rutile plates are required to be quite thick, each having a thickness of 1+.sqroot.2. These large crystals are costly and increase the overall size of the device.
An other optical isolator is described in U.S. Pat. No. 5,446,578 in the name of Chang et al. Chang et al. in FIG. 9A of the '578 patent illustrate a three stage optical isolator wherein a first and third crystal have a length t=a and wherein a second and fourth crystal have a length of .sqroot.2. Although this design overcomes the disadvantages of Kuzuta wherein each crystal is of a length 1+.sqroot.2, Chang et al propose a configuration, which introduces a different disadvantage. In FIG. 9A Chang et al. disclose the use of three Faraday rotators disposed between the four crystals. The second and third Faraday rotators 164 and 166 respectively are oppositely orientated such that rotator 164 non-reciprocally rotates light at -45.degree. where Faraday rotator 166 rotates light propagating through it at +45.degree.. Since the crystal between these two rotators is relatively thin, the oppositely oriented magnetic fields required to effect rotation of the two closely spaced rotators 164 and 166 interfere with each other and adversely effect the performance of the two rotators.
It is therefore an object of this invention to provide a multi-stage optical isolator having at least three stages that obviates the aforementioned disadvantages of Chang et al. and of Kuzuta.
It is therefore an object of this invention to provide an optical isolator that provides substantial isolation and which at the same time is relatively simple and cost effective to manufacture.
It is a further object of this invention to provide a multi-stage isolator that provides isolation for a relatively wide band of signals.